Pine wilt nematode

This is the view from the top of the track down to the larger building on the quinta. In many ways it encapsulates the nature of the “Green Heart of Portugal” – forested mountain ranges cut deep by meandering river valleys, peppered with tiny white villages perched on mountain ridges, surrounded by land terraced and richly cultivated with olives, vines, fruit trees, vegetables … Idyllic.

But it encapsulates something else about the Green Heart of Portugal too – an ecological disaster-in-the-making presently taking hold in Portugal’s forests. The tree on the left is dying.

Pinewood nematode, pine wilt nematode, pine wilt, pine wilt disease … all names given to the disease affecting rapidly growing numbers of the Maritime pines (Pinus pinaster) which form the vast bulk of Portugal’s forests. It’s caused by the nematode Bursaphelenchus xylophilus, a tiny worm that infests the sap wood of various species of pine, and is vectored through various wood-boring insects, depending on where it’s present. Native to North America Japan and Asia (see comments below), it has spread to the USA, Australia and Portugal, presumably via imported timber.

Not only does the nematode kill the trees it infects, it also renders the timber unusable. If left standing for more than a few months, the wood of the now-dead tree undergoes a change in structure and turns into a strange spongy substance that, aside from being useless for building, clogs the chainsaw blades that try to fell it and barely even burns, generating almost no heat.

When I first came here just 3 years ago, there was scarcely any evidence of it. People were talking about it appearing around Arganil, 20km away, but there was no obvious evidence of it in this valley. Now, this is the view across the valley.

The photo at the beginning of this post was taken last year, when we removed 3 trees with the disease on the quinta. This year we are going to have to take out more – I’ve counted around 10 and that’s before a thorough exploration of our bit of pine forest. Next year there will probably be more still …

So we’re bringing forward our plans for the woods and stepping things up a gear. The aim is to try and spot the infected trees at the first sign of infection and fell them before the wood becomes useless for firewood, burning them as soon as they’re dry, and stockpiling healthier trees for future years’ firewood, making clearings where we can start to plant more hardwoods. In doing so, we hope to steal a march on the disease. Lowering density and increasing diversity may offer some protection to remaining healthy trees, but even if it doesn’t, by the time the disease decimates the surrounding forest, we will hopefully have a well-established start to a new, more diverse and healthy forest.

My hopes are that it can also turn out to be a successful example of an alternative approach to the disease. Rather than clear-felling and replanting with eucalypts (now against the law, but flouted with virtual impunity see comments below) which will cause even more environmental damage and increase fire risk, I hope to show that it’s possible to regenerate a largely indigenous but also productive, valuable, non-flammable forest from which high-value timber such as chestnut can be harvested regularly through coppicing and provide a good return in less time than it would have taken to bring pine forest to maturity and harvest. Although we won’t be harvesting our trees in this way, for local people it’s part of their livelihood and how they look on their forest. If they’re to turn away from present practices, then there needs to be a reason and viable incentive to do so.

8 Comments

Hello from a silent reader! :)
I don’t know if you remember me, I sent you an email last year about some questions about moving to Portugal (thanks for your great answer, by the way!).
Very interesting post, I had never heard of this before. I wonder if there is anything you can interplant the pines with to fight the nematodes (like tagetes in your garden beds)…
Have a great day,
Luise

Hey Luise … yes I do! Glad it was helpful. Unfortunately there doesn’t seen to be any knowledge about companion plants that would help the pines. The disease isn’t a big problem in the United States, where it comes from, because indigenous pines there seem to have a natural resistance. It mainly affects imported ornamental pines (at least according to the USDA information leaflet I linked in the post). Most of the general information available on the web doesn’t seem to mention the susceptibility of the Maritime pine either. Or what happens to the timber after infection.

Here the nematode is mainly vectored by the pine sawyer or longhorn beetle Monochamus galloprovincialis.

The trees are infected by carrier beetles feeding on young shoots in the spring and summer. I would guess water stress and nutritional status could play a big part in how quickly the trees succumb, but it does seem that the Maritime pines here have no natural resistance to the nematode.

Hi there Just a couple of things to clarify …. just to let you know it Originated from Japan and Asia , NOT America, ,
it is not illegal to plant Eucalyptus you just need a permit (that is very easy to get ).
Also the Nematode now affects most species of Pinus including Pinus Brava , The exception is the ONLY native tree in Portugal the Stone pine , ( Pinus Pinea )

i did my dissertation on this …..and am now involved with research with the United Nation Forum on Forests.

Thanks for your input and corrections Steve! How did you manage to trace its origins to Japan? All the online references I consulted said USA, even the USDA! Maybe you could update the Wikipedia entry?

With the eucalypts, I thought you were only allowed to plant where eucalypts had already been growing? So it’s just a matter of a permit … so much for discouraging planting … :-(

I’m fascinated by the change in structure the wood undergoes once infected. Do you know if this is common to all infected species or just Maritime pine? And do you know exactly what happens to the wood?

Do you reckon there’s a chance of persuading both ordinary Portuguese forest-owners and the authorities that encouraging replanting of indigenous hardwoods, greater biodiversity and less reliance on highly flammable species could make a big difference to Portugal’s fortunes, both for individual landowners and as a nation?

How did you manage to trace its origins to Japan? All the online references I consulted said USA, even the USDA!

It was first traced from Japan Korea and China , it was found to have come in on shipping crates.
It was first found in Portugal in 1999 and has slowly spread through the Setúbal region near Lisbon. and then to the rest of the country, The disease has a high mortality rate for pine species in Europe (esp. Pinus pinaster and P. sylvestris) .

With the eucalypts, I thought you were only allowed to plant where eucalypts had already been growing? So it’s just a matter of a permit … so much for discouraging planting … :-(

Yes it is a joke , it was supposed to slow down the amount being planted , but obviously money talks, so you pay the money and you can plant , ( it is v cheap as well ), The only new rule that is enforced is that you have `safety` distances for planting next to roads and buildings.

I’m fascinated by the change in structure the wood undergoes once infected. Do you know if this is common to all infected species or just Maritime pine? And do you know exactly what happens to the wood?

Yes it is common to all the resinous trees, a very basic overview is that it dries the wood out ,as it kills the tree by cutting off the resin supply to the tree, ( this is why it always looks like they go yellow at the top first). it does a lot more as well but it is rather technical.

And do you reckon there’s a chance of persuading both ordinary Portuguese forest-owners and the authorities that encouraging replanting of indigenous hardwoods, greater biodiversity and less reliance on highly flammable species could make a big difference to Portugal’s fortunes, both for individual landowners and as a nation?

Yes it is already happening ,( slowly though and in a small scale !! ) a lot of people are now realising the benefits of Oak ,Chestnut and good Pines such as the native stone pine (Pinus pinea),
a lot of the problems on a larger scale are that the landowners have sold large tracts of land and forest that the big companies have bought and all they are interested in is making a profit fast.
70% of the EU based Carbon offsetting companies use Eucalyptus forests here in Portugal, as they get money twice, once from the company or individual who wants to feel better by offloading his Carbon onto someone else. and then again when you harvest every 6-10 years.
Also the pulp and papers companies can publicise that they are green and are planting 3 trees for every one they cut down, a good example is the EU toilet paper companies.

well all research and practical observation has noted that P. Pinea is resistant to the nematode,
it may have been simple normal `pine wilt `that affected your trees , rather than the nematode.
I certainly would be interested to know more about your trees.